UPDATE: we had a couple of googlers today (17 august 2010) from washington state searching for information about a loud noise they heard there today. below is the apparent reason:

Scrambled jets cause sonic booms in Puget Sound-area

by KING 5 News and Associated Press

KING5.com

Posted on August 17, 2010 at 2:23 PM

Updated today at 5:08 PM

SEATTLE - Military fighter jets were scrambled to respond to an airspace violation near Air Force One in Seattle on Tuesday afternoon, causing sonic booms that scared residents throughout Western Washington.

North American Aerospace Defense Command spokesman John Cornelio said the F15 jets were sent from Portland as the president was visiting Seattle after a report that an aircraft violated the restricted airspace. The small plane left the restricted air space before the jets arrived.

Pilot Lee Dailey and his passenger, Laura Joseph, were returning home after a weekend at Lake Chelan. They landed on Lake Washington in Kenmore.

"We just landed, sailed in here, nice and peaceful, pulled up to the dock and a guy came out and said I have some bad news for you guys," said Joseph. "Obama is in town and you guys violated the air space or something."

They reported that they saw jets as they flew between the Seattle area and the mountains, but said the jets didn't motion for them to land.

Once they landed, Dailey and Joseph were interviewed by the Secret Service. The Secret Service then spent about 90 mintues checking out the plane.

When the president is visiting there is a 10-mile radius from the center of Seattle and 18,000 feet up, where it is no-fly for commercial aircraft. Extended another 20 miles, to make it a 30-mile radius, there can be private air traffic if the pilot files a legitimate flight plan, if that person is in contact with air traffic control and if that person has the plane's transponder turned on.

There is no system for notifying private pilots about airspace restrictions. It's incumbent upon them to check to see if restrictions are in place.

911 call centers overloaded

Emergency agencies across Western Washington were flooded with calls from people who heard and felt a double boom about 1:50 p.m.

Residents in Pierce County ran from their homes.

"It shook my whole house," said Stephanie Walters of Lakewood.

"It rattled the whole house, the ground, everything," said Charlie Kuhn of Lakewood.

Det. Ed Troyer with the Pierce Co. Sheriff's Dept. said the 911 system was down via landlines in areas of Pierce County for a time and car and home alarms were set off by the booms.

KING 5 News was flooded with calls and e-mails from people who heard and felt the explosions. Many people reported that their homes shook.

The King County Sheriff's Office reported that about 300 calls came in to the 911 center. ~ source